AIMS The influence of alcohol on the brain serotonergic system has been studied for several decades with some discordant results. The effects of continuous and constant treatment with ethanol on the rates of serotonin [5-hydroxytryptamine (5-HT)] synthesis in discrete regions of the rat brain were studied. METHODS 5-HT synthesis rates were measured using the alpha-[(14)C]methyl-l-tryptophan autoradiographic method. The rats in the experimental group were treated with 50% ethyl alcohol and those in the control group received distilled water. The fluid was delivered subcutaneously by implanted osmotic mini-pumps for 14 days at the rate of 5 micro l/h or 0.12 ml/day (0.06 ml of alcohol per day). RESULTS Chronic ethanol treatment, as delivered in the present experiment, induced a significant increase in the rate of 5-HT synthesis in descending serotonergic cell bodies (raphe pallidum, raphe obscurus, raphe magnus), nigrostriatal structures, the hippocampus and cortices. No significant changes were observed in the dorsal and median raphe nuclei or pineal body. The results suggest that there may be differences in the regulation of 5-HT synthesis in different brain structures after 14 days of continuous (subcutaneous) injection of 50% alcohol. CONCLUSIONS Chronic ethanol treatments using osmotic mini-pumps induce non-uniform increases in 5-HT synthesis in the rat brain.